Frequency tracking loops are commonly employed in wireless systems to align a receiver frequency clock to a transmitter frequency clock, or to adjust received signals to account for frequency differences between a transmitter, e.g., in a wireless access network base station, and a receiver, e.g., in a mobile wireless device. Frequency tracking loops can be used to improve accuracy when recovering a signal from a noisy communication channel and can also be used to distribute clock timing information to properly align frequency sampling clock timing pulses in digital logic designs. Variability of a frequency of a crystal oscillator (XO) clock source in a receiver, however, can result in frequency errors with respect to a transmitted carrier frequency that can lead to a relative frequency error between the clock source used in the receiver of a mobile wireless device and a clock source used in a transmitter of a base station in a wireless access network. These frequencies errors can impact the performance of reception and decoding of the received signals in the mobile wireless device, and can thereby decrease the reliability and functionality of the wireless system.
Frequency errors can affect different types of wireless systems that use different radio access technologies, including code division multiple access (CDMA) wireless systems and orthogonal frequency division multiplexed (OFDM) wireless systems. In CDMA-based wireless systems, a frequency error can deteriorate the quality of received signals. Moreover, a carrier frequency error in the receiver can translate to sampling timing errors that can accumulate over time, which can break the orthogonality of spreading codes used in CDMA-based wireless systems to differentiate signals sent to different mobile wireless devices. Orthogonality of signals provided by code division multiplexing in CDMA-based wireless systems can be critical for a receiver in a mobile wireless device to separate signals intended for the mobile wireless device from signals intended for other mobile wireless devices. Loss of orthogonality can increase an amount of interference generated by “non-orthogonal” signals that can be simultaneously received with a signal intended for a particular mobile wireless device in a CDMA-based wireless system, thereby affecting received signal performance in the particular mobile wireless device. Similarly, a carrier frequency error can affect the orthogonality of different sub-channels used in an OFDM-based wireless system, and thereby can increase inter-channel interference (ICI) between sub-channels of the OFDM-based wireless system and deteriorate the overall performance of the OFDM-based wireless system.
Accurate and robust frequency estimation and frequency tracking can be critical for designing receivers for mobile wireless devices that reliably correct for frequency errors. Different frequency error estimation and tracking schemes can be used that provide a balance between accuracy, convergence time, and frequency pull-in range. A channel impulse response (CIR) can be estimated based on a set of received pilot symbols and/or a set of received data symbols, and the resulting CIR can be used to characterize phase information of a wireless communication channel between the mobile wireless device and the base station of a wireless access network. The phase information embedded in the CIR can include an amount of accumulated frequency error due to differences in clock frequencies at a source (e.g., the base station) and a sink (e.g., the mobile wireless device). CIR techniques can be accurate but require longer convergence time, higher computing power (which can result in higher power consumption), and narrower frequency pull-in range. Alternatively, a technique that exploits a continuous phase ramping between different samples of a symbol, e.g., due to frequency errors, can be used when frequency error estimation with a larger pull-in range than afforded by a CIR based frequency error estimation method is needed. Each frequency error estimation and tracking technique can be better suited to certain received signals depending on whether a higher accuracy, narrower pull-in range, “fine” frequency tracking loop or a lower accuracy, wider pull-in range, “coarse” frequency tracking loop is required.
Therefore, what is desired is an algorithm that combines fine frequency estimation results and coarse frequency estimation results and employs methods having different precision and complexity to improve both the accuracy and robustness of frequency estimation and frequency tracking in wireless systems.